The majority of electronic equipment throughout the world operates from alternating current voltage supplied by a utility company. In most such equipment, the supply voltage is inputted into a power supply, which might be of the increasingly popular line-operated AC to DC switching converter type. The supplied voltage may have different nominal voltage levels depending upon the part of the world in which the equipment is located. North American voltages are typically 115 volts, with tolerances which may allow it to decrease to 90 volts. In contrast, voltages in Japan are nominally 100 volts, but may decrease below 90 volts with tolerances included. Voltages in many European or African countries are nominally 220 volts, with tolerances allowing the voltage to rise above 260 volts. Moreover, the input voltage requirements expected of many DC to DC converters may be for wide ranges that even exceed that of AC line operated power supplies. Input ratios of as high as eight to one are currently in use.
Manufacturers of electronic equipment are finding it economically attractive to procure, stock and manufacture their product with a single power supply which can operate without adjustment from any nominal AC voltage found anywhere in the world. Thus, wide range input power supplies which can operate from 90 volts AC to 264 volts AC without adjustment are becoming highly desirable.
It is feasible to design a power supply of conventional technology to operate from 90 to 264 VAC (imposing essentially 100 to 375 volts DC from the rectifier section to the input of the DC to DC converter part of a line operated power supply). However, selection of the converter operating point to minimize conduction losses at the low voltage level requires the compromise of imposing high voltage stresses at the high voltage levels. This effect is especially severe with the so-called "buck derived" or forward converters, where the voltage stresses on the rectifiers and switching elements varies essentially as double the input voltage. Therefore, doubling the line voltage doubles the voltage stresses. A switching element with 200 volts stress at low line conditions must withstand about 750 volts at high line conditions. Components with optimized operating parameters over this wide range of voltage stress are too expensive for economical commercial power supplies. Thus, designers must compromise performance in order to utilize commercially priced components.
In view of the popularity of wide range input power supplies, and mindful of the voltage stresses in conventional topologies on switches and rectifiers which vary typically as twice the variation of the input voltage, a converter wherein the voltage stress changes with input change are less than linearly proportional to increases in the input voltage would be extremely useful.
The present invention teaches a novel bootstrap operation which allows the switching and rectifying elements of otherwise essentially familiar DC to DC converters to experience a greatly reduced voltage stress increase with increasing input voltage. In the present invention, the voltage stress on the switches and rectifiers varies directly with input change, plus a small essentially constant value that is equal to the magnitude of the voltage stress at minimum input voltage. In this manner, input voltage changes of eight to one result in a voltage stress increase of five to one, as opposed to an eight to one increase, as would be the case with a conventional forward converter.